hi davada my doubt is that MJL isnt a small transistor its a strong beast but at the same time mosfets also have advantages. So now which one to look at? When it comes to beta droop BJTs have beta droops and I have checked the graphs and found that the beta droop isnt much at 4Amps but when we use mosfets then I believe there is no current droop so would that help in delivering the steady current into the sub driver and hence giving better grip < provided that there is no much sag in the supply with large cap bank >.
The idea with the triple is not necessarily to get the output damping factor down.
200 is good enough. What we are concerned about with the triple is to unload the the high impedance output of the VAS. At high power the VAS loading become significant because of the beta drop of the output stage transistors. The output triple remedies this problem. One of the biggest concerns of VAS loading is the capacitance presented by the transistor cob. A triple allows for much smaller transistors with much lower cob interfacing to the VAS.
200 is good enough. What we are concerned about with the triple is to unload the the high impedance output of the VAS. At high power the VAS loading become significant because of the beta drop of the output stage transistors. The output triple remedies this problem. One of the biggest concerns of VAS loading is the capacitance presented by the transistor cob. A triple allows for much smaller transistors with much lower cob interfacing to the VAS.
And to bootstrap the collector voltages, which lowers the effective Cob even more.
Cheers,
E.
The Triple also provides much lower open-loop output impedance. This tends to allow the amplifier to be more stable with capacitive loads and/or a smaller value of inductance for the output coil. With a Triple, I have built amplifiers with a 1.4uH output coil in parallel with 2.2 ohms, and the amplifier was stable and well-behaved with a 0.1uF load right across the output. Square wave ringing was less than one full cycle and very well-damped. Ringing was at the resonant frequency of the 1.4uH coil and the 0.1uF load.
Cheers,
Bob
I think that a conventional class AB amplifier that needed 16 output pairs of either BJTs or MOSFETs would probably be rated on the order of 1200 W into 8 ohms or up to over 2000 W into 4 ohms. Is that really how much power you need for your subwoofer?
BTW, subwoofers can usually get away with an amplifier of lower sound quality. For this reason, among others, subwoofers these days usually employ a class D amplifier.
Cheers,
Bob
I wanted to understand how to drive number of mosfets.
Can you please tell me which driver to use to drive 14 or 20 pairs of IRFs
Yes I'm looking to drive a 21 inch pro sub with full grip.
I have a doubt reg sub amp with 5 pairs of MJL21194 / 93 or 10 pairs of IRF240/9240 to drive a sub at 600W into 4ohm? at +/-80V supply. I'm looking at super low frequency with full grip and control. Which one do you suggest and why?
Hi rhythmsandy,
You're going at this from the wrong direction completely! Right now your goal is to create and convert as much power as possible as heat energy. What you want is acoustic output at low frequencies. It would seem to me that you would be further ahead using woofers designed for efficiency at low frequencies. The lower the amount of energy you dump into a voice coil, the lower the temperature will be and the more energy you can create SPL with.
So, redefining what you actually want is the maximum air displacement with the minimum power utilized. Would that be accurate? I mean, if you could achieve the same end with 60 watts as you could with 1200 watts, which way would you go?
-Chris
You're going at this from the wrong direction completely! Right now your goal is to create and convert as much power as possible as heat energy. What you want is acoustic output at low frequencies. It would seem to me that you would be further ahead using woofers designed for efficiency at low frequencies. The lower the amount of energy you dump into a voice coil, the lower the temperature will be and the more energy you can create SPL with.
So, redefining what you actually want is the maximum air displacement with the minimum power utilized. Would that be accurate? I mean, if you could achieve the same end with 60 watts as you could with 1200 watts, which way would you go?
-Chris
My point is not about high spl with lower output power from the amp.
Im looking to drive the subwoofer without any stint. It just needs to hold the input sinewave. Consider a 21 inch pro subwoofer has moving mass of about 0.5Kg now imagine how much sustained power is required to drive this with full control. Most of the cases the amplifiers are capable of delivering the voltage into the load but current into the load that too very sustained current into the load is the question. Consider the butt kicker transducer which has a large steel moving piston mass of about close to 1.5Kg and that one works till 5Hz. The Class D amplifier which is built by butt kicker is burning out. Now lets consider that one as an example. Control till 5Hz for a moving mass of 1.5Kg and yet at 1200WRMS.
My question is simple when it comes to reactive loads yes we will have good current demand but my question is if the coil resistance is 3.5ohm then will it drop even further in a subwoofer? If so how much it would drop or it maintains the impedance curve?
coming to the beta droop of the transistors the calculations what I made for the 3.5ohm load at +/-70V hence 5 pairs of MJL21194/93 should be able to deliver the current into 3.5ohm load.
Why am I looking at mosfets: one primary reason is no beta droop hence the current demanded by load will be delivered without the current sag. But driving mosfets is a question. Apart from driving it which one is preferred going with 5 pairs of MJL or 10 pairs of IRF240/9240s?
Im looking to drive the subwoofer without any stint. It just needs to hold the input sinewave. Consider a 21 inch pro subwoofer has moving mass of about 0.5Kg now imagine how much sustained power is required to drive this with full control. Most of the cases the amplifiers are capable of delivering the voltage into the load but current into the load that too very sustained current into the load is the question. Consider the butt kicker transducer which has a large steel moving piston mass of about close to 1.5Kg and that one works till 5Hz. The Class D amplifier which is built by butt kicker is burning out. Now lets consider that one as an example. Control till 5Hz for a moving mass of 1.5Kg and yet at 1200WRMS.
My question is simple when it comes to reactive loads yes we will have good current demand but my question is if the coil resistance is 3.5ohm then will it drop even further in a subwoofer? If so how much it would drop or it maintains the impedance curve?
coming to the beta droop of the transistors the calculations what I made for the 3.5ohm load at +/-70V hence 5 pairs of MJL21194/93 should be able to deliver the current into 3.5ohm load.
Why am I looking at mosfets: one primary reason is no beta droop hence the current demanded by load will be delivered without the current sag. But driving mosfets is a question. Apart from driving it which one is preferred going with 5 pairs of MJL or 10 pairs of IRF240/9240s?
Hi David,
I have to agree. The choice of woofer is your main problem rhythmsandy. By choosing that woofer you are committing yourself to a path that quickly turns into a vicious circle. An intelligent person with a reasonable amplifier can (and would) blow your system out of the water.
rhythmsandy,
Now for the important question. Did you buy this speaker already? If so, good luck with this path. If not, then back up and look at what you are trying to do. Give it a few days and ask yourself what it is you are trying to accomplish here. A lighter paper coned woofer with long throw would be extremely efficient compared to that massive mis-step. Basically, if you can displace the same volume of air with a much more efficient radiator, you'll be laughing all the way to the bank in two ways.
1.) Repair costs as the huge amplifier will fail more often due to the high temperatures it will run at.
2.) The capital cost of all the components you are looking at.
If you want to examine a more realistic line, look at Electrovoice, Altec and JBL woofers. If EV is still around, they used to make some of the best drivers for bass. Another factor to seriously consider is how low the very room you're in will support in frequency. The diagonal from one corner on the floor to the opposite corner on the ceiling represents 1/2 of the wavelength of the lowest frequency your room will support. Be prepared to be depressed. Reality can really bite.
We are trying to save you a ton of money while also making your intended project a success.
-Chris
I have to agree. The choice of woofer is your main problem rhythmsandy. By choosing that woofer you are committing yourself to a path that quickly turns into a vicious circle. An intelligent person with a reasonable amplifier can (and would) blow your system out of the water.
rhythmsandy,
Now for the important question. Did you buy this speaker already? If so, good luck with this path. If not, then back up and look at what you are trying to do. Give it a few days and ask yourself what it is you are trying to accomplish here. A lighter paper coned woofer with long throw would be extremely efficient compared to that massive mis-step. Basically, if you can displace the same volume of air with a much more efficient radiator, you'll be laughing all the way to the bank in two ways.
1.) Repair costs as the huge amplifier will fail more often due to the high temperatures it will run at.
2.) The capital cost of all the components you are looking at.
If you want to examine a more realistic line, look at Electrovoice, Altec and JBL woofers. If EV is still around, they used to make some of the best drivers for bass. Another factor to seriously consider is how low the very room you're in will support in frequency. The diagonal from one corner on the floor to the opposite corner on the ceiling represents 1/2 of the wavelength of the lowest frequency your room will support. Be prepared to be depressed. Reality can really bite.
We are trying to save you a ton of money while also making your intended project a success.
-Chris
If you go with active or 6db/oct passive, minimum Z should not go lower then 3.5 ohm.
With 12db/oct passive topology minimum Z can go lower then Rdc of voice coil.
Consider the SOA for worst case reactance.... cos fi zero.
Which of course is not to say that you cannot play frequencies in that room below that 1/2 of the wavelength.
It just means that no standing waves can occur below that wavelength in that particular room, plus that you will get about 6 dB per octave room gain below that frequency. In other words, you will have more bass than you paid for.
Hoffman's Iron Law
In discussing how much power a subwoofer needs, it is useful to bear in mind what is called Hoffman's Iron Law in the loudspeaker community.
Hoffman's Iron Law refers to the relatively constrained relationship among low-frequency cut-off, sensitivity and enclosure size. We all would like to have all three: deep bass extension, high speaker sensitivity and small enclosure size. Hoffman's Iron Law says we can only have two out of the three.
For this reason, the numbers get ugly fast as you specify a lower cut-off frequency without making the enclosure huge. There are some fairly fundamental physics at work. On top of all of this, if you want high SPL at low frequencies, you have to move a lot of air. This means some combination of large cone area and large cone excursion.
Although vented (bass reflex) systems help in many situations and can provide very good bass extension down to, say, 30 Hz, their response drops like a rock below the vented box tuning frequency (24 dB/octave). It is also not practical to tune a reasonably-sized box to much below 30 Hz. As a result, subwoofers that are designed to go really deep often have to be of a closed-box (sealed enclosure) design. Such designs are fairly straightforward second-order spring-mass systems that often have something like a Butterworth response that drops at 12 dB/octave below the box tuning frequency. The dominating part of the "spring" is the compliance created by the cone area and the volume of the box. In these systems, the compliance spring of the cone by itself in free air usually does not dominate. A large cone in a small box creates a very tight spring.
Because the low-end cut-off frequency is set by the resonance of the woofer in the box, a tight spring means that the mass (which is largely that of the cone and voice coil) must be large to keep the resonant frequency down. This can literally mean that the cone wants to be heavier than needed. A light cone does not help here. A light cone will increase efficiency at higher frequencies, but that is not helpful in this kind of situation. The heavier cone needed to establish a low-enough resonant frequency in a reasonable sized enclosure results in lower sensitivity. This, in turn, results in the need for greater amplifier power for a given SPL.
If you are going to put a 21-inch woofer into a reaonable size box, lets say 8 cubic feet in a 2X2X2 cube, you must have significant mass to get a resonant frequency of, say, 20 Hz or lower. One can argue with the performance target that a person sets, but if they are crazy enough to want to go down below 20 Hz and achieve reasonably high SPL with low distortion in a reasonably-sized box, there is almost no way around needing high power.
If you go to, say, Parts Express, and look at their subwoofers, you will indeed see examples of subwoofer designs with very high power amplifiers.
So, for a high-performance subwoofer, amplifier power in excess of a kilowatt is certainly not out of the question. Bob Carver's Sunfire subwoofer is also an example of this sort of thing. At some point, it just sort of ends up being brute force, where the brute force is the amplifier power.
I would still recommend using a reliable class-D amplifier for this application. This is especially true if, as with most subwoofers, you want the amplifier integrated in the box as part of the subwoofer - getting rid of the heat can be a huge problem if one uses a class AB amplifier as a plate amp in a subwoofer.
Cheers,
Bob
In discussing how much power a subwoofer needs, it is useful to bear in mind what is called Hoffman's Iron Law in the loudspeaker community.
Hoffman's Iron Law refers to the relatively constrained relationship among low-frequency cut-off, sensitivity and enclosure size. We all would like to have all three: deep bass extension, high speaker sensitivity and small enclosure size. Hoffman's Iron Law says we can only have two out of the three.
For this reason, the numbers get ugly fast as you specify a lower cut-off frequency without making the enclosure huge. There are some fairly fundamental physics at work. On top of all of this, if you want high SPL at low frequencies, you have to move a lot of air. This means some combination of large cone area and large cone excursion.
Although vented (bass reflex) systems help in many situations and can provide very good bass extension down to, say, 30 Hz, their response drops like a rock below the vented box tuning frequency (24 dB/octave). It is also not practical to tune a reasonably-sized box to much below 30 Hz. As a result, subwoofers that are designed to go really deep often have to be of a closed-box (sealed enclosure) design. Such designs are fairly straightforward second-order spring-mass systems that often have something like a Butterworth response that drops at 12 dB/octave below the box tuning frequency. The dominating part of the "spring" is the compliance created by the cone area and the volume of the box. In these systems, the compliance spring of the cone by itself in free air usually does not dominate. A large cone in a small box creates a very tight spring.
Because the low-end cut-off frequency is set by the resonance of the woofer in the box, a tight spring means that the mass (which is largely that of the cone and voice coil) must be large to keep the resonant frequency down. This can literally mean that the cone wants to be heavier than needed. A light cone does not help here. A light cone will increase efficiency at higher frequencies, but that is not helpful in this kind of situation. The heavier cone needed to establish a low-enough resonant frequency in a reasonable sized enclosure results in lower sensitivity. This, in turn, results in the need for greater amplifier power for a given SPL.
If you are going to put a 21-inch woofer into a reaonable size box, lets say 8 cubic feet in a 2X2X2 cube, you must have significant mass to get a resonant frequency of, say, 20 Hz or lower. One can argue with the performance target that a person sets, but if they are crazy enough to want to go down below 20 Hz and achieve reasonably high SPL with low distortion in a reasonably-sized box, there is almost no way around needing high power.
If you go to, say, Parts Express, and look at their subwoofers, you will indeed see examples of subwoofer designs with very high power amplifiers.
So, for a high-performance subwoofer, amplifier power in excess of a kilowatt is certainly not out of the question. Bob Carver's Sunfire subwoofer is also an example of this sort of thing. At some point, it just sort of ends up being brute force, where the brute force is the amplifier power.
I would still recommend using a reliable class-D amplifier for this application. This is especially true if, as with most subwoofers, you want the amplifier integrated in the box as part of the subwoofer - getting rid of the heat can be a huge problem if one uses a class AB amplifier as a plate amp in a subwoofer.
Cheers,
Bob
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